East Antarctic sea ice: Albedo, thickness distribution, and snow cover

Abstract

Characteristics of springtime sea ice off East Antarctica were investigated during a cruise of the Australian National Antarctic Research Expedition in October through December 1988. The fractional coverage of the ocean surface, the ice thickness, and the snow cover thickness for each of several ice types were estimated hourly for the region near the ship. These observations were carried out continuously during the 4 weeks the ship was in the ice. Thin and young ice types were prevalent throughout the region, and the observations show a systematic increase in the total area‐weighted pack ice thickness (including open water area) from only 0.2 m within 50 km of the ice edge to 0.45 m close to the coast. Ice thickness averaged over the ice‐covered region only is also relatively thin, ranging from 0.35 m near the ice edge to 0.65 m in the interior. These values are probably typical of average winter thickness for the area. The average snow cover thickness on the ice increased from 0.05 m near the ice edge to 0.15 m in the interior. Average ice concentration increased from less than 6/10 near the ice edge to 8/10 in the interior. The ship‐observed concentrations were in good agreement with concentrations derived from passive microwave satellite imagery except in some regions of high concentration. In these regions the satellite‐derived concentrations were consistently lower than those estimated from the ship, possibly because of the inability of the satellite sensors to discriminate the appreciable percentage of very thin ice observed within the total area. Spectral albedo was measured for nilas, young grey ice, grey‐white ice, snow‐covered ice, and open water at wavelengths from 420 to 1000 nm. Allwave albedo was computed by using the spectral measurements together with estimates of near‐infrared albedo and modeled spectral solar flux. Area‐averaged albedos for the East Antarctic sea ice zone in spring were derived from representative allwave albedos together with the hourly observations of ice types. These area‐averaged surface albedos increased from about 0.35 at the ice edge to about 0.5 at 350 km from the edge, remaining at 0.5 to the coast of Antarctica. The low average albedo is in part due to the large fraction of open water within the pack, but extensive fractions of almost snow‐free thin ice also play an important role.